Counterfeit detection apparatus is typically utilized to determine the genuiness of paper money and the like. For example, certain regions of U.S. paper currency are printed with an ink containing magnetizable particles. Counterfeit currency typically does not use such ink. Alternatively in some instances counterfeit currency utilizes too great an amount of magnetic particles or alternatively utilizes a reproduction technique in which toner material which is similar to that used in photocopier machines and having a high concentration of magnetic particles is used to produce the currency.
One detection technique presently in use is described in U.S. Pat. No. 4,114,804 and comprises a permanent magnet member and a magnetic sensor and associated circuitry, which elements are integrated into a high speed currency counter. A stack of bills are placed in the currency counter infeed hopper. When the apparatus is turned on, the paper currency is bottom fed one sheet at a time through a combined feed/stripper assembly. Each bill passes through the magnetic field created by the permanent magnet, causing those ferro-magnetic particles on the bill which pass through the magnetic field to be magnetized. The magnetized particles which pass the sensor cause the generation of an electric signal which fluctuates as a function of magnetic field strength. The generated signal is typically non-uniform due to the rather random distribution of the ferromagnetic particles on the bill, but is generally characterized as an alternating or a.c.-type signal.
The signal generated by the sensor is rectified and is compared against a reference level signal. The circuitry associated with the sensor halts the currency counting operation in the event that the detected signal fails to reach the predetermined threshold. So long as the detected signal exceeds the predetermined threshold, the counting operation continues undisturbed.
The gap between each single fed bill is utilized by the counting apparatus for counting purpose and is also utilized to initiate a halting operation when the trailing edge of a bill passes the magnetic sensor.
In order to enhance the sensitivity and accuracy of the detection operation, this inventor developed a circuit described in U.S. patent application Ser. No. 524,856 filed Aug. 19, 1983. The improved circuitry described in the aforementioned pending patent application utilizes a sensing means for sensing the presence of a magnetic field, which sensing means further comprises an integral band pass circuit which passes only those detected signal lying within a narrow predetermined pass band. The signal lying within the pass band is rectified and compared against a reference level. The result of the comparison is temporarily stored in binary form, one binary state representing a good bill and the remaining binary state representing a suspect bill. When the trailing edge of the bill just examined passes the magnetic sensor, the stored condition is examined to generate a signal representative of the type of bill examined.
Although the above-mentioned counterfeit detection apparatus operated satisfactorily, it is desirable to provide counterfeit detection apparatus having greater sensitivity and reliability for use in currency counting apparatus having a wide range of operating speeds.